/* Upcall routine, designed to work as a key type and working through
 * /sbin/request-key to contact userspace when handling DNS queries.
 *
 * See Documentation/networking/dns_resolver.rst
 *
 *   Copyright (c) 2007 Igor Mammedov
 *   Author(s): Igor Mammedov (niallain@gmail.com)
 *              Steve French (sfrench@us.ibm.com)
 *              Wang Lei (wang840925@gmail.com)
 *              David Howells (dhowells@redhat.com)
 *
 *   The upcall wrapper used to make an arbitrary DNS query.
 *
 *   This function requires the appropriate userspace tool dns.upcall to be
 *   installed and something like the following lines should be added to the
 *   /etc/request-key.conf file:
 *
 *      create dns_resolver * * /sbin/dns.upcall %k
 *
 *   For example to use this module to query AFSDB RR:
 *
 *      create dns_resolver afsdb:* * /sbin/dns.afsdb %k
 *
 *   This library is free software; you can redistribute it and/or modify
 *   it under the terms of the GNU Lesser General Public License as published
 *   by the Free Software Foundation; either version 2.1 of the License, or
 *   (at your option) any later version.
 *
 *   This library is distributed in the hope that it will be useful,
 *   but WITHOUT ANY WARRANTY; without even the implied warranty of
 *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See
 *   the GNU Lesser General Public License for more details.
 *
 *   You should have received a copy of the GNU Lesser General Public License
 *   along with this library; if not, see <http://www.gnu.org/licenses/>.
 */

#include <linux/module.h>
#include <linux/slab.h>
#include <linux/cred.h>
#include <linux/dns_resolver.h>
#include <linux/err.h>
#include <net/net_namespace.h>

#include <keys/dns_resolver-type.h>
#include <keys/user-type.h>

#include "internal.h"

/**
 * dns_query - Query the DNS
 * @net: The network namespace to operate in.
 * @type: Query type (or NULL for straight host->IP lookup)
 * @name: Name to look up
 * @namelen: Length of name
 * @options: Request options (or NULL if no options)
 * @_result: Where to place the returned data (or NULL)
 * @_expiry: Where to store the result expiry time (or NULL)
 * @invalidate: Always invalidate the key after use
 *
 * The data will be returned in the pointer at *result, if provided, and the
 * caller is responsible for freeing it.
 *
 * The description should be of the form "[<query_type>:]<domain_name>", and
 * the options need to be appropriate for the query type requested.  If no
 * query_type is given, then the query is a straight hostname to IP address
 * lookup.
 *
 * The DNS resolution lookup is performed by upcalling to userspace by way of
 * requesting a key of type dns_resolver.
 *
 * Returns the size of the result on success, -ve error code otherwise.
 */
int dns_query(struct net *net,
              const char *type, const char *name, size_t namelen,
              const char *options, char **_result, time64_t *_expiry,
              bool invalidate)
{
        struct key *rkey;
        struct user_key_payload *upayload;
        const struct cred *saved_cred;
        size_t typelen, desclen;
        char *desc, *cp;
        int ret, len;

        kenter("%s,%*.*s,%zu,%s",
               type, (int)namelen, (int)namelen, name, namelen, options);

        if (!name || namelen == 0)
                return -EINVAL;

        /* construct the query key description as "[<type>:]<name>" */
        typelen = 0;
        desclen = 0;
        if (type) {
                typelen = strlen(type);
                if (typelen < 1)
                        return -EINVAL;
                desclen += typelen + 1;
        }

        if (namelen < 3 || namelen > 255)
                return -EINVAL;
        desclen += namelen + 1;

        desc = kmalloc(desclen, GFP_KERNEL);
        if (!desc)
                return -ENOMEM;

        cp = desc;
        if (type) {
                memcpy(cp, type, typelen);
                cp += typelen;
                *cp++ = ':';
        }
        memcpy(cp, name, namelen);
        cp += namelen;
        *cp = '\0';

        if (!options)
                options = "";
        kdebug("call request_key(,%s,%s)", desc, options);

        /* make the upcall, using special credentials to prevent the use of
         * add_key() to preinstall malicious redirections
         */
        saved_cred = override_creds(dns_resolver_cache);
        rkey = request_key_net(&key_type_dns_resolver, desc, net, options);
        revert_creds(saved_cred);
        kfree(desc);
        if (IS_ERR(rkey)) {
                ret = PTR_ERR(rkey);
                goto out;
        }

        down_read(&rkey->sem);
        set_bit(KEY_FLAG_ROOT_CAN_INVAL, &rkey->flags);
        rkey->perm |= KEY_USR_VIEW;

        ret = key_validate(rkey);
        if (ret < 0)
                goto put;

        /* If the DNS server gave an error, return that to the caller */
        ret = PTR_ERR(rkey->payload.data[dns_key_error]);
        if (ret)
                goto put;

        upayload = user_key_payload_locked(rkey);
        len = upayload->datalen;

        if (_result) {
                ret = -ENOMEM;
                *_result = kmemdup_nul(upayload->data, len, GFP_KERNEL);
                if (!*_result)
                        goto put;
        }

        if (_expiry)
                *_expiry = rkey->expiry;

        ret = len;
put:
        up_read(&rkey->sem);
        if (invalidate)
                key_invalidate(rkey);
        key_put(rkey);
out:
        kleave(" = %d", ret);
        return ret;
}
EXPORT_SYMBOL(dns_query);